Question on leadership

I posted this in the leadership group too.

As I continue my search for a position in Educational Leadership one question keeps haunting me that has been brought up three times during interviews. How can you as an instructional leader improve a mediocre teacher's performance?

That's an interesting and extremely difficult question to answer and I've given two different answers to the same question.

What can one do to improve a mediocre teacher's performance? In the first interview I answered: Through observation and professional development. However, as a teacher myself I'm always doing the song and dance when being observed. Even a mediocre teacher is going to do the dog and pony show.

The second and third time I answered: I suggested that an instructional leader should look at student achievement and data in the classroom to pin point specific weaknesses. However, I don't know if that's the correct answer either as some student's just don't perform as well as others and the teacher can't always be blamed for poor student outcomes. There are so many other variables involved than just looking at data to improve teacher performance.

How would you as an instructional leader, principal, a.p., director, dept. chair, or lead teacher answer this question?